Neurohypophysial disorders

Question 1

How does the PPg appear in MRIs?

Answer 1

A bright spot- always use MRIs to visualise pituitary gland

Question 2

What hormones come from the PPG?

Answer 2

Oxytocin

Vasopressin (ADH)

Question 3

APG and PPG types of tissue

Answer 3

APG- glandular tissue

PPG- neural tissue

Question 4

How does vasopressin act?

Answer 4

Vasopressin is an antidieuretic. It acts to increase water reabsorption from renal cortical and medullary collecting ducts (via V2 receptors).

When vasopressin binds to the V2 receptor of the collecting duct cell, it acts so that AQP2 aquaporins are inserted to the lumen side (apical) of the membrane. There is increased water uptake into the cell and therefore into the plasma (through V3 and V4 receptors).

Water travels in across osmotic gradient and the net result is water reabsorption.

Question 5

What regulates vasopressin release

Answer 5

Osmoreceptors

Question 6

Where are the osmoreceptors located?

Answer 6

Organum vasculosum (near the hypothalamus)

They project through the supraoptic nucleus and the paraventricular nucleus.

Question 7

How do the osmoreceptors chnage in shape?

Answer 7

Osmoreceptors are really sensitive to changes in extracellular osmolality. An increase in extracellular Na+ increases the osmolality of the surrounding extracellular environment. Water leaves the osmoreceptors, down the concentration gradient and the osmoreceptors shrink.

Question 8

What is the signifcance of the osmoreceptor shrinking?

Answer 8

The shrinking increases osmoreceptor firing.

Vasopressin is released from hypothalamic paraventricular nucleus and the supraoptic nucleus. Nuclei are groups of neuronal cell bodies.

Plasma osmolality is restored.

Question 9

What is diabetes insipidus?

Answer 9

Absence or lack of circulating vasopressin. You cannot reabsorb water from the collecting duct

Question 10

What are the 2 types of diabetes insipidus?

Answer 10

1. Cranial
2. Nephrogenic (end organ resistance to vasopressin)

Question 11

Describe cranial (acquired) diabetes insipidus

Answer 11

Occurs due to damage of the hypophysial system.

Reasons for this happening include:

• Traumatic brain injury
• Pituitary surgery
• Pituitary tumours, craniopharyngioma (brain tumour derived from pituitary tissue)
• Metastasis to the pituitary glad (e.g. from breast)
• Granulomatous infiltration of median eminence e.g. TB, sarcoidosis- stalk issues issues between the communication of the hypothalamus and the PPG.

Question 12

Congenital cranial diabetes insipidus

Answer 12

RARE

Question 13

Congential nephrogenic diabetes insipidus

Answer 13

If it does occur, it is due to a mutation in the gene coding for any of the receptors/ proteins in the aquaporin pathway (e.g. in the V2 receptors)

Question 14

Acquired nephrogenic diabetes insipidus

Answer 14

Due to drugs (like lithium)

Question 15

General signs and symptoms of diabetes insipidus

Answer 15

• Large volumes of urine (polyuria)
• Dilute urine (hypo-osmolar)
• Thirst and increased drinking (polydipsia)
• Dehydration (and consequences) if fluid intake is not maintained it can lead to death
• Disruption during sleep with these problems

Question 16

What is psychogenic polydipsia?

Answer 16

Patients present with the same symptoms as someone with DI.

Seen in psychiatric patients- may reflect anti-cholinergic effects of medication (dry mouth).

Also seen in patients who are told to drink a lot of fluids.

They may have Excess fluid intake (polydipsia) and excess urine output (polyuria) but they still have the ability to secrete vasopressin in response to osmotic stimuli is preserved.